Summary Reserpine in dosage sufficient to lower brain stem 5-hydroxytryptamine and norepinephrine suppresses endotoxin fever in rabbits. Pretreatment of rabbits with agents depleting only brain stem norepinephrine but not 5-hydroxytryptamine does not suppress febrile responses to endotoxin. An unexpected finding was the depletion of norepinephrine from the brain stem by guanethidine.